Don't invalidate paint on background image change if it's obscured

third_party/WebKit/LayoutTests/fast/repaint/obscured-background-no-repaint.html

Index: third_party/WebKit/LayoutTests/fast/repaint/obscured-background-no-repaint.html
diff --git a/third_party/WebKit/LayoutTests/fast/repaint/obscured-background-no-repaint.html b/third_party/WebKit/LayoutTests/fast/repaint/obscured-background-no-repaint.html
index c3157a55f2c8d1fd016db8c00019bbc171110295..8c785ad89e98c6cfc2f14ce408636f80d5d3983d 100644
--- a/third_party/WebKit/LayoutTests/fast/repaint/obscured-background-no-repaint.html
+++ b/third_party/WebKit/LayoutTests/fast/repaint/obscured-background-no-repaint.html
@@ -51,7 +51,7 @@
     window.testIsAsync = true;
     function repaintTest()
     {
-        runAfterLayoutAndPaint(finishRepaintTest);
+        setTimeout(finishRepaintTest, 300);

[Stephen Chennney, 2016/08/09 14:22:31]

Timeouts should be avoided if at all possible. They lead to test flakiness and
add to trybot time. Is there any way to restructure this to avoid the timeout?

[Xianzhu, 2016/08/09 18:51:48]

Unfortunately there seems no other ways for such tests containing animated gifs.
The test is designed to be stable: even if the test is slow causing the animated
gif not to update frames in time, it will still produce the expected result (no
paint invalidation).

     }
 
     function start() {
@@ -59,7 +59,9 @@
             return;
 
         var img = new Image();
-        img.onload = runRepaintTest;
+        img.onload = function() {
+          runAfterLayoutAndPaint(runRepaintTest);
+        };
         img.src = "resources/animated.gif";
     }
 </script>